1. Field of the Invention
The present invention relates to a corrosion-resistant film for protecting surfaces of silver (Ag), with protection from being corroded by ozone, sulfide components, etc. in air, to a composite structure composed of a Ag substrate and the above-mentioned corrosion-resistant film, which is used as a reflective film, for example, for a photo-magnetic recording medium, an optical recording medium, a reflector, an illuminator, a sign, etc. and as a plating film, for example, for a metal product, a plastic product, etc, and to a method of protecting an Ag surface from corrosion from sulfide components and ozone in air.
2. Discussion of the Background
Ag is an extremely useful metal, having a beautiful silver-white gloss and excellent ductility and malleability and having the largest electric conductivity and thermal conductivity in the group of metals. In particular, since Ag has a reflectivity of almost 100% against visible rays in a broad wavelength range, it is widely used as a reflective film for various devices needing mirror surfaces, such as in optical recording media, etc. If tableware is plated with Ag, they have improved outward appearance and microorganisms in water attached to them are killed. Ag-plated utensils have a high-quality feel and their commercial value is elevated.
As mentioned above, Ag is an extremely useful metal, which is chemically stable in that it is not oxidized even when heated at high temperatures in oxygen. However, one drawback of Ag is that it is converted into brown or black Ag.sub.2 S in the presence of sulfide components in air or black AgO in the presence of ozone in air. The latter is not so remarkable as the former, with the result being that Ag shall lose its gloss. In particular, Ag directly bonds to sulfur and, for example, it reacts with H.sub.2 S to be easily converted into Ag.sub.2 S and blackened even at room temperature. While oppositely utilizing the blackening reaction of Ag, Ag decorations with antique colors are known but are not popular. In order to temporarily prevent such a blackening reaction of Ag, methods of treating Ag with chromates and of coating Ag with transparent resins are known. A more effective method in which Ag is plated with a thin rhodium (Rh) film is also known.
However, all of the above-mentioned known methods for preventing Ag from fading, are not satisfactory. Precisely, the chemical film to be formed on Ag by the treatment with chromates is extremely thin and it loses its activity within a relatively short period of time. It is difficult to completely remove film defects such as pin holes, etc. from the resin film to be formed on Ag. If the resin film formed on Ag has such defects, Ag is corroded through the defects. The Rh-plating method is not popular since Rh itself is very expensive. In addition, when an Ag film is used as the reflective film for a photo-magnetic recording media, etc. the thermal conductivity of the protective film is often problematic.
The present inventors have studied means of protecting Ag from being corroded by sulfide components and ozone in air and, as a result, have found that the corrosion of Ag can be effectively prevented when an Ag substrate is coated with an Ag--Mg alloy. Such an Ag--Mg alloy film has a larger residual compression stress than an Ag film and is therefore harder than the latter. However, if such an Ag--Mg alloy film is formed on a substrate or base layer, the adhesion between the film and the substrate or base layer is poor so that the former is easily peeled off from the latter. If so, the substrate or base layer is corroded through the part from which the alloy film has been peeled off. Therefore, only if an Ag film is simply substituted with such an Ag--Mg alloy film, a sufficient protecting effect can not be obtained. The present inventors have further studied the problem and, have found that when an Ag--Mg alloy film is formed on an Ag substrate as its protective film while forming an integrated structure of the Ag substrate and the Ag--Mg alloy film, that an Ag composite structure with extremely good corrosion resistance can be obtained. On the basis of these findings, we have completed the present invention.